A federal judge has ordered the Food & Drug Administration to stop antibiotics from being used to fatten up food-producing animals, unless such practice can be proven safe. The decision comes in response to a lawsuit filed last May by the Natural Resources Defense Council and other advocacy groups that believe excessive use of antibiotics in farm animals contributes to a rise in antibiotic-resistant bacteria, thereby threatening human health.

The lawsuit alleged that FDA acted illegally by not following through with its 1977 proposal to ban the use of penicillin and the tetracycline class of antibiotics for promoting growth in livestock. At that time, FDA said such uses could not be shown to be safe. But “FDA never held hearings or took any further action on the proposed withdrawals,” U.S. Magistrate Judge Theodore H. Katz wrote in his March 22 decision.

“In the intervening years, the scientific evidence of the risks to human health from the widespread use of antibiotics in livestock has grown, and there is no evidence that the FDA has changed its position,” Katz said.

Katz directed FDA to prohibit the addition of penicillin and tetracyclines to animal feed for growth promotion, unless drug manufacturers can provide evidence at a public hearing that such uses are safe. The antibiotics can still be used, however, for disease treatment and prevention in livestock.

Rep. Louise M. Slaughter (D-N.Y.), who has long pressured FDA to address overuse of antibiotics in farm animals, applauded the ruling. “This is a good first step,” she said, “but to really get in front of this problem we must address all classes of antibiotics in farm animals.”